I am sorry for that screwing you got. I think I would document all of this, and notify your credit card company that there is an issue that you will be trying to work out with the merchant.
Lay a thick paper trail. Document everything you can including if you can get it someone that is an "expert" giving an opinion on how the engine differed in spec from what you bought.
You may even want to ID the new tenant people in Baldwin Park and document their story.
Once you have the technical information, write the guy a letter laying out your case. If you have documented it well enough that he realizes he has no wiggle room, he MAY help you out. If not write a letter to the credit card company with your correspondence with him , a copy of your bill with the charges highlighted and a description of the issue that you have tried to work out IN GOOD FAITH with the merchant.
The credit card company will usually credit the charges pending disposition and investigation of the story.
I have done it before and it worked every time. You may even get the credit card to revoke his Credit Charging privelages as a merchant. If he is as big a crook as it sounds he is...
Good luck man...'
Oh yeah and send any letters fed ex, ups or airborn so you have a paper trail that he got it. Post office is OK with certified return receipt but there is no way to check on the delivery on the internet.